Stein Industrie's published French patent No. 2 507 778 (equivalent to U.S. Pat. No. 4,438,784) relates to a plug of this type, in which the first component includes an inside thread and the second component is an inner component having a solid end, having an outside thread which is screwed into the inside thread of the first component, which is lip-welded at its top edge or outside edge to said first component, and which includes internal means enabling it to be unscrewed.
Such a plug makes it possible to inspect welds in the pipework or the apparatus numerous times by means of an X-ray source inserted into the pipework or the apparatus by unscrewing the second component, and then by screwing it back into place after the weld has been inspected.
However, it is not possible to perform satisfactory lip-welding between the first and second components when the pipework or the apparatus is made of a high strength steel alloy since such alloys are very liable to crack after heat treatment, in particular after welding.
Preferred implementations of the present invention provide a plug for an opening providing an inspection X-ray source with access to pipework or to apparatus made of high-strength steel alloy and enabling a weld in said pipework or said apparatus to be inspected many times in spite of the susceptibility of such steels to cracking.